Disc drive memory systems have been used in computers for many years as storage space for digital information. Information is recorded on concentric memory tracks of magnetic discs that rotate around a spindle. Information is accessed by read/write heads located on a pivoting arm which moves radially over the surface of the disc. The read/write heads--transducers--must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
The discs are rotated at high speeds in an enclosed housing by means of an electric motor located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft in the hub. One of the bearings is located near the top of the spindle and the other near the bottom. These bearings allow for rotational movement between the shaft and the hub while maintaining accurate alignment of the spindle and the shaft. The bearings are normally lubricated by grease or oil.
The conventional bearing system described above is prone, however, to several shortcomings. First, vibration is generated by the balls rolling on the raceways. Ball bearings used in hard disk drive spindles run under conditions that often cause physical contact between raceways and balls in spite of the lubrication layer provided by the bearing oil or grease. Hence, ball bearings running on the apparently even and smooth, but microscopically uneven and rough, raceways transmit surface and circular imperfections in the form of vibration to the rotating disk. This vibration results in misalignment between the data tracks and the read/write transducer. These imperfections thus limit data track density and the overall performance of the disc drive system, as well as limiting the lifetime of the disc drive.
Another problem is related to the use of hard disk drives in portable computer equipment and the resulting requirements in shock resistance. Shocks create relative acceleration between the disks and the drive casting. Since the contact surface in ball bearings is very small, the resulting contact pressures may exceed the yield strength of the bearing material and leave permanent deformation and damage on raceways and balls.
Moreover, mechanical bearings are not always scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
Another problem is that of potential leakage of grease or oil into the atmosphere of the disc drive, or outgassing of the components into this atmosphere. Because of the extremely high tolerance for smaller spaces between tracks on the disc and the gap in the transducer which is used to read and write data on the disc, discs are located within sealed housings in which contaminants cannot be tolerated.
Another problem is the friction that is generated on the ball bearing surface. Since ball bearings are always in mechanical or physical contact with the lubricating layer of grease or the motor shaft, resulting friction limits the maximum possible speed of rotation for acceptably long life. This limitation conflicts with the need to spin the magnetic disc at ever higher speeds in order to improve the overall performance of the disc drive. Moreover, such conventional spindle motors are prone to spindle bearing motion or run-out. As a result of such run-out, the disc that is rigidly mounted to the rotating spindle may tilt or wobble, especially when assaulted by the shocks which are inherently imposed on the small disc drives utilized in portable computer systems. Since such bearings have a non-repeatable runout, a limit on track pitch or TPI is imposed.
As an alternative to conventional ball bearing spindle systems, researchers have worked on developing a hydrodynamic bearing. In these types of systems, lubricating fluid (gas or liquid) functions as the bearing surface between a stationary base housing and the rotating spindle/hub. For example, liquid lubricants including oil, more complex ferro-magnetic fluids, or even air have been utilized in hydrodynamic bearing systems. Air is popular because it is important to avoid the outgassing of contaminants into the sealed area of the head disc housing. However, air cannot provide the lubricating qualities of oil or the load capacity. Its low viscosity requires smaller bearing gaps and therefore higher tolerance standards to achieve similar dynamic performance. Thus an air bearing with adequate stiffness becomes expensive for most operating speeds of less than 6000 rpm.
The liquid lubricant must be sealed within the bearing to avoid a loss which would result in reduced bearing load capacity and life. Otherwise, the physical surfaces of the spindle and of the housing would come into contact with one another leading to accelerated wear and eventual failure of the bearing system. Equally serious, the failure of such a seal or other effort to contain the lubricant within the bearing system would allow contaminants entry into the head disc region of the disc drive.
In the prior art, seals for containing the fluid within the disc drive utilize a pressurized film on the surface of the liquid air interface, or capillary action. In the case of bearing assemblies which employ ferro-magnetic fluids, the seal is achieved by means of a magnetic field established at each end of the bearing.
Other obvious shortcomings of known hydrodynamic bearings include the fact that many prior art hydrodynamic bearing assemblies require large or bulky structural elements for supporting the axial and radial loads, as such hydrodynamic bearings do not have the inherent stiffness of mechanical bearing assemblies. It is difficult to scale the structural support elements to fit within the smaller disc drive dimensions currently in demand. In other instances, hydrodynamic bearing assemblies require extremely tight clearances and precise alignments. This burden makes it difficult to manufacture such assemblies since even a small deviation or aberration can lead to faulty bearings.
A combination of a magnetic bearing and a hydrodynamic bearing has been proposed. This would potentially solve a start torque and possible contamination/particle count issue that could otherwise develop when the hydrodynamic bearing is at rest. Contact between the stationary and rotating members is constant until the spinning portion develops enough pressure to lift off and stop rubbing. The magnetic bearing provides a centering force so that contact of parts does not occur at rest and there is no rubbing. However, use of such a combination of bearings adds complexities to the design of a hydrodynamic bearing.
As mentioned previously, the stiffness of the hydrodynamic bearing is also a major issue. The stiffer the bearing, the higher the natural frequencies in the radial and axial directions, and the track on which reading and writing occurs is more stable. Therefore, it is advantageous to use a fluid or grease which has a viscosity significantly greater than air. However, it is also true that this fluid must be contained so that it does not contaminate the drive, and the running torque for drag is increased with the fluid, but damping of motion occurs. This damping of motion must be balanced against the fact that the fluid viscosity is temperature dependent and so stiffness, damping and drag are functions of temperature. These functions can vary by an order of magnitude over the expected temperature range of interest --5.degree. C. to 70.degree. C.--following the known phenomenon that viscosity is reduced as temperature is elevated.
Thus the stiffness of a bearing having a defined gap and its ability to support a load or accurately support a disc rotating on a spindle without wobble or tilt will be decreased as the temperature of the fluid in the hydrodynamic bearing is increased.